1. Field of the Invention
The present invention relates to a multi-color image forming apparatus such as an electrophotographic apparatus and, more particularly, to a multi-color image forming apparatus in which a plurality of color component images are sequentially transferred and superimposed on a recording medium carried by a conveying belt.
2. Description of the Related Art
In an image forming apparatus such as a multi-color printer or a multi-color copy machine, a plurality of image forming units are serially arranged along a conveying belt, the image forming units form color component toner images corresponding to yellow, magenta, cyan and black. Each of the color component images are transferred and superimposed on a transfer sheet conveyed by a conveying belt so that a multi-color or full-color image is formed. In the above-mentioned image forming apparatus such as an electrophotographic apparatus, it is required to accurately superimpose color component images without an offset with respect to each other so as to form a high-quality color image.
Japanese Laid-Open Patent Application No. 6-18796 discloses an image forming apparatus which corrects a color offset with respect to a reference color (black, for example) by forming register marks corresponding to color component images on a conveying belt and detecting the register marks by a CCD sensor.
Additionally, Japanese Laid-Open Patent Application No. 8-123129 discloses an image forming apparatus similar to the image forming apparatus disclosed in the above-mentioned patent document. The image forming apparatus disclosed in Japanese Laid-Open Patent Application No. 8-123129 further comprises a stain preventing member which prevents formation of a stain on the register marks.
Each of the above-mentioned conventional image forming apparatuses is structured as shown in FIG.1. That is, an image forming unit 20Y, an image forming unit 20M, an image forming unit 20C and an image forming unit 20K are arranged along a conveying belt 35 which is drivingly engaged with a drive roller 36 and an idle roller 37. The image forming units 20Y, 20M, 20C and 20K form a yellow toner image, a magenta toner image, a cyan toner image and a black toner image, respectively.
Additionally, a paper supply cassette 40 which stores transfer papers is provided under the conveying belt 35. A paper supply roller 41 which feeds the transfer paper is provided on an end portion of the paper supply cassette 40. A register roller 42 which feeds the transfer paper to the conveying belt 35 is provided near the image forming unit 20Y. A fixing roller 43 and a pressing roller 44 which fix a toner image formed on the transfer paper are provided near the drive roller 36.
The image forming unit 20Y comprises a photosensitive drum 1Y, a charger 30Y, an optical writing unit 31Y, a developing unit 32Y, a transfer unit 33Y, and a cleaning unit 34Y. The charger 30Y charges the photosensitive drum 1 so that an electrostatic latent image is formed on the photosensitive drum 1Y by the optical writing unit 31Y. The developing unit 32Y develops the latent image as a yellow (Y) toner image. The Y toner image is transferred to transfer paper. The cleaning unit 34Y removes toner remaining on the photosensitive drum 1Y.
Similarly, the image forming unit 20M comprises a photosensitive drum 1M, a charger 30M, an optical writing unit 31M, a developing unit 32M, a transfer unit 33M, and a cleaning unit 34M. The image forming unit 20C comprises a photosensitive drum 1C, a charger 30C, an optical writing unit 31C, a developing unit 32C, a transfer unit 33C, and a cleaning unit 34C. The image forming unit 20K comprises a photosensitive drum 1K, a charger 30K, an optical writing unit 31K, a developing unit 32K, a transfer unit 33K, and a cleaning unit 34K.
In the above-mentioned structure, a position offset sensor 45 is provided near the drive roller 36. The position offset sensor 45 detects register marks formed by the image forming units 20Y, 20M, 20C and 20K. A discharger 38 is provided on the downstream side of the position offset sensor 45 so as to discharge the conveying belt 35. A cleaning unit 39 is provided near the idle roller 37 so as to remove toner remaining on the conveying belt 35.
In the above-mentioned conventional image forming apparatus, the Y toner image is transferred onto a transfer paper by the image forming unit 20Y so that the Y toner image is transferred in synchronization with the conveyance of the transfer paper by the transfer belt 35. The transfer paper having the Y toner image is conveyed to a position corresponding to the image forming unit 20M. Then, a magenta (M) toner image is transferred and superimposed on the Y toner image by the image forming unit 20M. Similarly, a cyan (C) toner image is transferred and superimposed on the M toner image and, then, a black (K) toner image is transferred on the M toner image. Accordingly, a multi-color or full-color image is formed by the superimosingly transferred Y toner image, M toner image, C toner image and K toner image being superimposed. The multi-color image is fixed on the transfer paper by being passed through a portion between the fixing roller 43 and the pressing roller 44.
In the above-mentioned image forming process, register marks corresponding to each color of the image forming units 20Y, 20M, 20C and 20K are formed and developed on an area of each of the photosensitive drums 1y, 1M, 1C and 1K, respectively. The register marks are transferred to the conveying belt 35 in synchronization with a transfer operation of each of the Y, M, C and K toner images by the respective transfer units 33Y, 33M, 33C and 33K. Then, the register marks in each color are read by the position offset sensor 45 so as to detect an offset of the register marks corresponding to Y, M and C with respect to K. A writing position of each of the optical writing units 31Y, 31M, and 31C is adjusted so as to correct the offset detected by the position offset sensor 45.
In the above-mentioned conventional image forming apparatus, since the endless conveying belt 35 is driven by the drive roller 36, speed of the conveying belt 35 periodically fluctuates due to an eccentricity of the drive roller 36 or an eccentricity of rotational force transmitting parts such as a gear for transmitting a rotational force to the drive roller 36.
When such a periodic fluctuation occurs in the speed of the conveying belt 35, the register marks are formed at positions slightly offset from accurate positions in which the register marks are to be formed since the operation of forming the register marks is performed on the assumption that the conveying belt 35 is moving at a constant speed. Accordingly, the register marks are detected by the position offset sensor 45 at slightly offset positions. Thus, there is a problem in that an accurate detection of the offset in the positions of the register marks cannot be performed due to the periodic fluctuation in the speed of the conveying belt 35.
A description will now be given of another conventional image forming apparatus in which a color offset is corrected by detecting a position offset of a register mark corresponding to each color component image.
FIG. 2 is an illustration of a structure of a conventional color image forming apparatus. In FIG. 2, parts that are the same as the parts shown in FIG. 1 are given the same reference numerals, and descriptions thereof will be omitted. The color image forming apparatus shown in FIG. 2 has the same structure with the image forming apparatus shown in FIG. 1 except for the position offset sensor 45 being replaced with a register mark detecting sensor 14 located on the same side where the image forming units 20Y, 20M, 20C and 20K are located.
In the color image forming apparatus shown in FIG. 2, a recording paper (transfer sheet) 10 is fed onto the conveying belt 35 from the paper cassette 40. The recording paper 10 is secured on the conveying belt 35 by an electrostatic force, and conveyed to the image forming unit 20Y so that an yellow toner image is formed on the recording paper 10. Thereafter, a magenta toner image, a cyan toner image and a black toner image are sequentially and formed and superimposed by the respective image forming units 20M, 20C and 20K. After the black toner image is formed by the image forming unit 20K, the recording paper 10 is passed through the fixing unit comprising the fixing roller 43 and the pressing roller 44 so that the toner image on the recording paper 10 is fixed, and then the recording paper 10 is ejected to a paper eject tray (not shown in the figure). It should be noted that operations of the optical writing units 31Y, 31M, 31C and 31K are controlled by a control unit 53 so that the Y, M, C and K toner images are accurately formed on the respective photosensitive drums 1Y, 1M, 1C and 1K.
FIG. 3A is a perspective view of a part of the color image forming apparatus shown in FIG. 2. In FIG. 3A, a direction indicated by an arrow B (hereinafter referred to as direction B) is perpendicular to a moving direction of the conveying belt 35 indicated by an arrow C (hereinafter referred to as direction C). That is, the direction B corresponds to a primary scanning direction, and the direction C corresponds to a secondary scanning direction. In the color image forming apparatus, if a distance between the image forming units 20Y, 20M, 20C, and 20K or an angle of each of the image forming units 20Y, 20M, 20C and 20K is shifted from a correct position, this causes a color offset (an offset in a registration of color component images) in the output image and results in deterioration of the output image quality. Accordingly, in the color image forming apparatus, each of the image forming units 20Y, 20M, 20C and 20K forms a register mark 15 on the conveying belt 35 so that an offset in a registration of color component images can be detected. The correction is performed based on the offset in the registration of each of the color component images by detecting the register mark 15 formed by each of the image forming units 20Y. 20M, 20C and 20K. The register mark 15 and the register mark detecting sensor 14 are shown in FIG. 3A. The register marks 15 are formed on each side of the conveying belt 35. Thus, the register mark detecting sensor 14 is provided on each side of the conveying belt 35 on the downstream side of the image forming unit 20K so as to detect the register marks 15 formed on the conveying belt 35.
FIG. 3B is a perspective view of the register mark detecting sensor 14. Each of the register marks 15 comprises a mark extending in the direction B perpendicular to the direction C of the movement of the conveying belt 35 and a mark inclined a predetermined angle (for example, 45 degrees) with respect to the direction B. Each of the register mark detecting sensors 14 is located in a position where the register marks 15 can be detected. Hereinafter, a description will be give to one of the register mark detecting sensors 14 since they are identical to each other. The register mark detecting sensor 14 detects a time when the register mark 15 passes the position of the register mark detecting sensor 14. The register position offset of each register mark 15 is obtained based on the time of passage of each register mark 15.
The register mark detecting sensor 14 comprises a light-emitting diode (LED) 14-1, a slit plate 14-2 and a light-receiving element 14-3. The LED 14-1 is located on the side of the conveying belt 35 where the register mark 15 is formed so as to project a light to the register mark 15. The slit plate 14-2 and the light-receiving element 14-3 are located on the opposite side of the conveying belt 35, that is, an inner side of a loop formed by the conveying belt 35. The slit plate 14-2 has a slit having a shape the same as that of the register mark 15 so that the light projected from the LED 14-1 passes therethrough. The light-receiving element 14-3 receives the light passing through the slit of the slit plate 14-2. Accordingly, the light-receiving element 14-3 receives the light projected from the LED 14-1 when the register mark 15 is not present. On the other hand, the light-receiving element 14-3 receives a reduced light when the register mark 15 passes directly above the slit plate 14-2. The light-receiving element 14-3 detects the time when the register mark 15 passes by a difference in the amount of received light.
FIG. 4A is an illustration showing a positional relationship between the register mark detecting sensor 14 and the register mark 15 comprising a pair of marks K1 and K2 formed by the image forming unit 20K (black) and a pair of marks C1 and C2 formed by the image forming unit 20C (cyan). When the mark K1 or C1 is aligned with the slit extending in the direction B, or when the line mark K2 or C2 is aligned with the slit inclined with respect to the direction B, an amount of light received by the light-receiving element 14-3 is minimized. FIG. 4B is a time chart showing a peak of a detection signal output by the register mark detecting sensor 14. The peak indicates a time when the amount of light received by the register mark detecting sensor 14 is minimized. Accordingly, time TK1, TK2, TC1 and TC2 correspond to time when the corresponding marks K1, K2, C1 and C2 pass the register mark detecting sensor 14.
An offset of a register position of the cyan toner image with respect to a reference color toner image (black, in this case) can be obtained by the following relationship, where V0 is a speed of movement of the register mark 15, that is, a speed of movement of the conveying belt 35; and T0 is a time difference between the time when the mark K1 is detected and the time when the mark C1 is detected. It should be noted that an angle of the marks K2 and C2 with respect to the respective mark K1 and C1 is 45 degrees.
An amount E of the offset of a position of the cyan tone image in the primary scanning direction (direction B) with respect to the reference color toner image (black) is represented by the following relationship. EQU E={(TC2-TC1)-(TK2-TK1)}.times.V0 (1)
An amount F of the offset of the position of the cyan toner image in the secondary scanning direction (direction C) with respect to the reference color toner image (black) is represented by the following relationship. EQU F={(TC2-TC1)-T0)}.times.V0 (2)
A description will now be given of a more specific example. It is now assumed that the cyan marks C1 and C2 are spaced from the respective line marks K1 and K2 by a distance of 30 mm in the secondary scanning direction so that the mark K2 (black) does not cross the mark C1 (cyan). Accordingly, if the marks C1 and C2 are shifted toward the marks K1 and K2 by the distance of 30 mm, the line marks C1 and C2 coincide with the respective marks K1 and K2. That is, the cyan marks C1 and C2 do not have a position offset with respect to the black marks K1 and K2.
In FIGS. 4A and 4B, if V0=100 mm/sec; TK1=0 sec; TK2=0.1 sec; TC1=0.3 sec; TC2=0.4 sec; and T0=0.3 sec, this means that a distance between marks K1 and K2 is 10 mm; a distance between marks K1 and C1 is 30 mm; and a distance between marks K1 and C2 is 40 mm. In this condition, an amount of offset of position in the primary scanning direction and the secondary scanning direction can be calculated by the above relationships (1) and (2) as follows. EQU E={(0.4-0.3)-(0.1-0)}.times.100=0 mm EQU F={(0.3-0)-0.3}.times.100=0 mm
As appreciated from above, no offset of position is present in both the primary scanning direction and the secondary scanning direction.
FIGS. 5A and 5B correspond to FIGS. 4A and 4B, respectively, in a case when an offset of position is generated in both the primary scanning direction and the secondary scanning direction. It should be noted that, in FIGS. 5A and 5B, the offset of position is emphasized for the sake of easy recognition.
In FIGS. 5A and 5B, if V0=100 mm/sec; TK1=0 sec; TK2=0.1 sec; TC1=0.301 sec; TC2=0.4015 sec; and T0=0.3 sec, this means that a distance between marks K1 and K2 is 10 mm; a distance between marks K1 and C1 is 30.1 mm; and a distance between marks K1 and C2 is 40.15 mm. In this condition, an amount of offset of position in the primary scanning direction and the secondary scanning direction can be calculated by the above relationships (1) and (2) as follows. EQU E={(0.4015-0.301)-(0.1-0)}.times.100=0.05 mm=50 .mu.m EQU F={(0.301-0)-0.3}.times.100=0.1 mm=100 .mu.m
As appreciated from the above, the amount E of the offset of position in the primary scanning direction is 50 .mu.m, and the amount F of the position offset in the secondary scanning direction is 100 .mu.m.
As mentioned above, the position offset of each color register mark with respect to the reference color register mark can be calculated by detecting the time when each register mark 15 passes the register mark detecting sensor 14. Accordingly, an appropriate correction can be performed for a timing of the image forming operation so as to achieve an accurate registration of the register position.
The above mentioned calculation of the amount of position offset is based on the assumption that the speed V0 of movement of the conveying belt 35 is constant. However, in practice, there is a fluctuation in the speed of movement of the conveying belt 35 due to a fluctuation in a rotational speed of the drive roller or an eccentricity of the circumference of the drive roller with respect to the rotational axis thereof. If the speed of the conveying speed fluctuates, an error may be generated in the calculated amounts E and F of the offset of position.
FIG. 6 is a graph of a speed V of movement in which a periodic fluctuation is generated. In FIG. 6, an average speed V0 of movement of the conveying belt 35 is 100 mm/sec, and a periodic fluctuation of about .+-.0.2 mm/sec is generated.
Consideration is given to a case in which the above-mentioned marks K1, K2, C1 and C2 are detected when the periodic fluctuation is generated in the speed of movement of the conveying belt 35 as shown in FIG. 6. A positional relationship between the marks K1, K2, C1 and C2 is the same as that shown in FIG. 4A. That is, the distance between marks K1 and K2 is 10 mm; the distance between marks K1 and C1 is 30 mm; and the distance between marks K1 and C2 is 40 mm. Thus, if the marks C1 and C2 are shifted toward the marks K1 and K2 by the distance 30 mm, the marks C1 and C2 coincide with the respective marks K1 and K2.
In FIG. 6, a time t when the mark K1 is detected zero (t=0), the speed V(t) of movement of the conveying belt 35 is represented by the following relationship. EQU V(t)=V0+V1.times.cos (.omega.t) (3)
Where, V0=100 mm/sec; V1=0.2 mm/sec; and .omega.=2.pi./1.2 rad/sec.
Additionally, a length L(t) of the conveying belt which passes the register mark detecting sensor 14 can be calculated by integrating the speed of movement L(t) with respect to the time. The result of the integration is as follows. EQU L(t)=V0.times.t+(V1/.omega.).times.sin (.omega.t) (4)
With respect to the time when the marks K2, C1 and C2 are detected, the time should satisfy the condition such as L(t)=10 mm; L(t)=30 mm; and L(t)=40 mm. For example, this condition is satisfied if TK1=0 sec; TK2=0.09981 sec; TC1=0.29962 sec and TC2=0.39967 sec. Additionally, the amount of position offset is obtained by the relationships (1) and (2) as follows. EQU E=0.024 mm=24 .mu.m EQU F=-0.038 mm=-38 .mu.m
As mentioned above, although the register marks shown in FIG. 4A are supposed to have no position offset in either the primary scanning direction or the secondary scanning direction, there is a detection error due to a fluctuation in the moving speed of the conveying belt that cannot be neglected. That is, there is a problem in that an error is generated due to a fluctuation in the moving speed of the conveying belt when an amount of position offset is calculated by detecting the register mark on the conveying belt.